A tracer study was performed to assign a radical produced with UV irradiation on photocatalytically active BaTi4O9 for water decomposition. BaTi4O9 was reduced in H-2 and oxidized in O-16(2), O-17(2) or O-18(2) at different temperatures. The behavior of oxygen exchange was investigated by thermal gravimetric analysis and Raman spectroscopy. Electron magnetic resonance spectra showed that new signals with g (perpendicular to) = 2.038 and g (perpendicular to) = 2.026 due to hyperfine splitting of O-17(-) with A (perpendicular to) = 19.1 G appeared when oxidized in O-17(2), in addition to normal signals with g (perpendicular to) = 2.018 and g(parallel to) = 2.004. The radical is assigned to O- produced from surface lattice oxygen of BaTi4O9.